1. Field of the Invention
The present invention relates generally to miniaturized camera products. More specifically, the present invention relates to a combined actuator that drives both shutter and focus functions in miniaturized camera products.
2. Description of the Related Art
This section is intended to provide a background or context. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the claims in this application and is not admitted to be prior art by inclusion in this section.
The traditional shutter driver for a camera forces constant current to be drawn through a magnetic actuator. To have a predictable and stable closing time, the current needs to be an accurate DC current. The actuation principle is similar for aperture adjust and neutral density (ND) filter actuators. The magnetic actuator has equivalent resistance and inductance. Generally, current needed for the shutter driver is between 60 mA and 200 mA. The direction of the current determines the opening or closing of the shutter.
FIG. 1 illustrates a circuit diagram of a traditional driver for a camera shutter and iris-ND filter. To close the shutter, switch S1 is closed and S2 is open. To close the iris-ND filter, switch S5 is closed and S6 is open. In either situation (closing the shutter or closing the iris-ND filter), switch S4 is closed and S3 is open. When S4 is closed, an operational amplifier 20 forces the voltage over a resistor 22 to be equal to the reference voltage, Vref by controlling the gate voltage of G1 or G5 depending upon whether the shutter or iris is opened. The gate voltage defines the channel resistance of the MOSFET, which in turn, defines the current. The closed loop control system sets the current to the Vref/R. The resistor is often an external 1% accurate resister. A one Ohm resistor is a typically selection.
To open the shutter, switch S1 is opened and S2 is closed. To open the iris-ND filter, switch S5 is opened and S6 is closed. In either situation (opening the shutter or opening the iris-ND filter), switch S3 is closed and S4 is open when S2 or S6 is closed (depending whether shutter or iris is operated). An operational amplifier 20 forces the voltage over a resistor 22 to be equal to the reference voltage, Vref, by controlling the gate voltage of G3. The gate voltage defines the channel resistance of the MOSFET, which in turn defines the current. The closed loop control system sets the current to the Vref/R. In this configuration, current flows in a different direction (compared to the closing situation).
Generally, the shutter actuator has a resistance of about 8-24 Ohms and requires current from about 60 mA to 200 mA. Where the resistance is 8 Ohms and the current is 200 mA, the voltage over the actuator is 1.6 V. About 0.2 V is needed over the resistor. Mobile cameras typically have a supply voltage of 2.8 V. Thus, only 1 V is left to be divide over switch S1 and S4. It follows that the resistance of S1 and S4 should be 2.5 Ohms, which is a large area low Ron MOSFET. Currently, the trend in mobile devices is to reduce the supply voltage, which would require even larger MOSFETS.
Class D amplifiers are typically used to drive zoom and autofocus actuators. These actuators can be piezoceramic actuators. A class D amplifier is an amplifier in which the output transistors are operated as switches rather than as a current source. Because an ideal switch has either zero voltage across it or zero current through it at all times, it dissipates no power. When a particular transistor is turned off, the current through it is zero. When the transistor is turned on, the voltage across the switch is small (ideally zero). This increases the overall efficiency of the amplifier, requiring less power from the power supply and smaller heat sinks for the amplifier.
A conventional tuned class D type of amplifier is shown in FIG. 2(a). A class D amplifier includes a P-type FET (PFET), an N-type FET, (NFET) two body diodes and a tuned load. The two FET gates are driven with signals that are about identical, but that prevents a simultaneous state of the two FET gates and a large shoot through current flow from the supply to ground. Another version of class D amplifier is shown in FIG. 2(b), where a rectangular waveform is applied to a low-pass filter rather than a tuned filter. The low-pass filter allows only its slowly-varying DC or average voltage to appear on the load. In FIG. 2(b), separate VP and VN drive stages are shown. These can be used to make the drive signals for VP and VN such that a simultaneous ON state for the two semiconductor switches can be prevented. The circuit shown in FIG. 2(a) can also have such driving of the gates. Although reasonably useful, class D amplifiers suffer from significant drawbacks. The major factors limiting the performance of class D inverters are switching losses and switching noise as discussed below. The switching loses result at least partially from resistive losses in Ron of switching devices. As such, it is advantageous to have low Ron devices driving piezo actuators. Although, in contrast to class A, B, and C amplifiers, switched mode power amplifiers such as class D amplifiers have an idealized efficiency of 100%, the combination of switching and conduction losses sets an upper bound on the amplifiers' power efficiency.
U.S. patent application Ser. No. 11/205,558, filed Aug. 17, 2005 and which is assigned to the same assignee as the present application, provides for the use of class DE amplifiers in conjunction with piezoceramic elements for actuating digital camera systems such as autofocus and zoom lens systems. In class DE amplifiers, switching losses are reduced in comparison to class D amplifiers. Each switching transistor in a class DE amplifier is on for less than a half period. There are two intervals of time in a period when both of the transistors are simultaneously off. During these intervals of “under lapping,” the shunt capacitances are recharged by the load current from 0 to Vmax or from Vmax to 0. As such, each transistor is turned on under its output voltage Vout≈0. Therefore, the switching power losses are substantially absent. In addition, electromagnetic interference is reduced because of “soft switching” during the dead time of the switches.
FIG. 3 illustrates a conventional combination of a shutter-iris driver and an autofocus/zoom actuator driver. The combination includes an iris-neutral density (ND) filter 32, a shutter 34, a piezo actuator 36, and a piezo actuator 38. The iris-ND filter 32 is coupled to MOSFETS 42, 44, 46, and 48. The shutter 34 is coupled to MOSFETS 46, 48, 50, and 52. The piezo actuator 36 is coupled to MOSFETS 54 and 56. The piezo actuator 38 is coupled to MOSFETS 58 and 60. As such, regardless of whether a class D or class DE amplifier is used, the conventional combination of a shutter iris driver and zoom/autofocus functions needs 10 MOSFETS and 5 outputs (outputs 61-65).
Thus, there is a need for a combination actuator that saves four MOSFETS and two outputs, resulting in six MOSFETS and 3 outputs. Further, there is a need for a combination actuator that results in smaller used silicon space and reduced costs to manufacture.